N!

[Trent Moseley]

Some time back there was a discussion on this Forum about the squareroot of pi divided by two equalling one-half factoral or 0.8862+. How is it that on some HP machines (67, 12C, 42S) the factoral function works only for integers and on others (15C, 32Sii) it works for fractions as well?

tm

[Ellis Easley]

Some HP calculators, including the 34C, use the same key for both the factorial and gamma functions. The best explanation I have read is from Viktor Toth:

http://www.rskey.org/gamma.htm

[hugh]

indeed, some hps let x! = gamma(x+1)
also the 48g has this, but not the 20s.

interestingly, the only other machine i've seen to handle non-integer factorials is the sharp el-9900 (current production unit). which can handle x! for frac(x) = 0 or frac(x) = 0.5

this means .5! = sqrt(pi)/2 and you can calculate values for the chi-square, F & T distributions, since these only use half factorials.